Hot strip mill



Aug. 9, 1966 C. P. LAYARD 3,%@,@

HOT STRIP MILL Filed Sept. 23, 1963 5 Sheets-Sheet L I IIZZ H :mmB: H H

N INVENTOR CAMVILLE P. LAYARD ATTORNEYS.

G. P. LAYAIRE) HOT STRIP MILL Aug; 9, 1966 5 Sheets-Sheet 2 Filed Sept.23, 1963 MmVFZE 9am DDQQQOQ OAUQQ INVE NTOR CAMVILLE P. LAYARD ATTORNEYS.

9, 1956 a. P. LAYAR HOT STRIP MILL Filed Sept. 23, 1963 :5 Sheets-Sheet:5

INVENTOR CAMVILLE P LAYARD ATTORNEYS.

United. States Patent 3,264,856 HOT STRIP MILL Camville Pellew Layard,Burlington, Qntario, Canada, assignor to The Steel Company of Canada,Limited, Hamilton, Ontario, Canada Filed Sept. 23, 1963, Ser. No.310,779 Claims priority, application Canada, June 26, 1963,

878,798 7 Claims. (Cl. 72-202) This invent-ion relates to a hot stripmill of the type used in the steel industry.

In the making of steel strip, a slab of steel is heated in a furnace toabout 2200 F. The slab may be, for example, 5 tons in weight, feet long,37 inches wide and 6- inches thick, although in some the slabs areconsiderably larger. The hot slab slides out of the furnace on runnersor skids to a conveyor where it is conveyed on rollers to a roughingmill. In a semi-continuous hot strip mill the roughing mill consists ofa single stand of rollers through which the slab makes 3 to 5 or morepasses back and forth to be rolled into a strip about A of an inch thickand 150 to 200 feet long. In a continuous hot strip mill the roughingmill may consist of, for example, six stands of rollers through whichthe slab passes, one stand after the other, in the same direction. Asthe strip emerges from the rollers of the roughing mill, the strip isblasted with a jet of water to be de-scaled. The jet of water isdirected mainly against the direction of advance of the strip so thatmost of the bits of scale fly backward, although some of the scale doesfly forward. From the roughing mill the strip advances along a delaytable on a series of conveyor rollers to the finishing train. Thefinishing train may conslot of six stands of rollers which roll thestrip to its final gauge. Located just in front of the finishing trainis a crop shear which cuts the crops and squares the end of a strip, anda scale breaker which removes any scale which may have formed while thestrip was on the delay table before the strip enters the finishingtrain. The finished strip which emerges from the finishing train may heof an inch thick and 800 to 1800 feet long or longer. The strip iscoiled up into a large coil or roll ready for further processing.

There are three main disadvantages associated with a hot strip mill ofthe type described above, all three of these disadvantages having to dowith cooling of the strip before it reaches the finishing train. It isdesirable to ensure that the temperature of the strip is greater thanabout 1550- F. while the strip is in the finishing train to avoid coldWorking of the metal with corresponding changes in its hardness. Itfrequently happens in prior art hot strip mills that the temperature ofat least the tail end of the strip (which remains on the delay tablelonger than the head end) falls below the critical value of l550 F.

It is also desirable to ensure that the temperature of the tail end ofthe strip as it enters the finishing train approximates as closely aspossible to the temperature at which the leading end entered thefinishing train. As mentioned above, the tail end of the strip remainson the delay table longer than the head end. The result with the priorart hot strip mills is that the tail end cools for a longer time to amuch lower temperature. As metal cools it becomes harder and in order tomaintain a reasonably constant thickness of the strip as it cools, moreforce must be exerted by the rollers in the finishing train. The forceexerted by the rollers is controlled by strain gauges which measure thestrain in the roller housings and X-ray gauges which monitor thefinished product. The strain gauges are responsive to both the thicknessand the hardness of the strip passing through the rollersan increase ineither requiring an increase in force by the rollers to keep thethickness constant. The force applied by the 3,254,856 Patented August9, i966 rollers to the strip is exerted on the rollers at their ends. Asthe force is increased to overcome the increasing resistance of thestrip due to the differential cooling of the head and tail ends of thestrip, the rollers, which are confined only at their ends, bow outwardlyat the middle resulting in crowning along the center of the strip.Crowning in itself is undesirable since it is important that theproperties of the strip-including thickness as Well as hardness-beconstant. Further disadvantages of crowning are that center buckling andedge tear frequently result during subsequent processing of the strip.Thus the second disadvantage associated with prior art hot strip millsis differential cooling of the strip before it enters the finishingtrain.

The third disadvantage is the occurrence of skid marks in the strip.Skid marks are areas of the strip which are cooler than the adjacentareas on either side. Since the color of a hot strip is determined byits temperature, the skid marks, being cooler, appear as dark patches.Skid marks result from differential heating and are troublesome for thereasons given in the last paragraph. However the dilferential heatingresulting in skid marks is caused quite differently from the continuousdifferential coo-ling from head to tail of the strip due to thedilfcrential time of expo-sure of the head and tail before entering thefinishing train. Skid marks occur when the hot slab of metal slidesthrough the furnace on water cooled runners or skids to the conveyorwhich conveys the slab to the roughing mill. The water cooled skidsconduct heat away from the areas of the slab in contact with the skids,and these areas remain cooler than their adjacent areas and are visibleas skid marks even after passing through the roughing mill. The skidmarks are greatly lengthened by the roughing mill, as is the rest of thestrip, but the skid marks stay in the strip as it enters the finishingtrain.

These three difiiculties a temperature of some parts of the strip below1550 F. in the finishing train, a non- .constant temperature in thefinishing train, and skid marks-ore all very well known in the steelindustry, and attempts have been made to overcome at least the first twodisadvantages.

One attempt to maintain the temperature of the strip above 1550" F. atthe finishing train has been to increase the temperature of the furnace,thereby giving the strip a higher initial temperature. However it hasbeen found that the life of the furnace is greatly reduced if thefurnace is operated above about 2400 F.

Another attempt-this one an attempt to solve both the problem of thestrip temperature falling below 1550 F. and the problem of differentialcooling between the head and the tail of the strip-has been to shortenthe time the strip'is on the delay table. This has been accomplished bythe introduction of Zoom rolling techniques by which the speed of thestrip is increased as it travels through the mill. However Zoom rollingrequires a considerable increase in the power of the mill in order toachieve effective speeds. Furthermore, Zoom rolling introduces problemsin the synchronization of the coilers that coil up the finished strip.The greater the speeds that are attained, the greater the problems inthe synchronization.

The present invention overcomes, or at least mitigates, all three of theabove disadvantages. The invention in its broadest aspect comprises amethod of reducing the cooling of a bar of metal in a hot strip millcomprising the step of reducing the rate of loss of heat due toradiation from at least one surface of the bar. The invention alsocontemplates apparatus for carrying out the above method. In oneembodiment of the apparatus of the present invention, a radiationreflector is provided which is adapted to be positioned in the vicinityof the bar or strip to reflect back at least some of the radiationradiated from the strip. In this embodiment, the reflector is preferablya sheet of aluminum and forms a concave surface about the upper surfaceof the strip, thus focussing the reflected radiation more effectivelyback towards the strip.

In another and more preferred embodiment of the apparatus of the presentinvention, a heat shield is provided in the vicinity of, and insubstantially parallel relation to, the hot strip during at least partof the travel of the strip through the hot strip mill. The heat shieldis adapted to quickly absorb some of the heat radiated from the stripand to remain hot from the previous bar thereby resulting in anincreased and relatively high temperature of the shield and hence of thesurroundings of the strip, so that further radiation from the strip willbe reduced in accordance with the Stefan-Boltzmann law:

Q is the heat radiated per unit area per unit time K is a constant T isthe absolute temperature of the strip T is the absolute temperature ofthe surroundings.

Without the shield the difference in temperature between the strip andits surroundings is great (of the order of 2000 F. or 1000 K.), and theamount of heat radiated per unit time is considerable. In the time ittakes for the strip to reach the finishing train, the total amount ofheat lost-and the corresponding decrease in the temperature of thestriprnay be quite extensive. However, with the second type of heatshield, the difference between the temperature of the strip and itssurroundings is substantially reduced with a corresponding reduction inthe radiation and hence in the temperature drop.

The heat shield can be in the form of a tunnel covering the upper sufaceof the strip. In a semi-continuous hot strip mill the shield can be usedover the delay table between the roughing mill and the finishing train;in a continuous mill the shield can also be used between the individualstands of rollers in the roughing mill. The heat shield canadvantageously be made of sheet stainless or other steel with a backinglayer ofinsulating asbestos. In normal use the shield might bepositioned with its steel surface facing the strip about 4 /2 inchesabove it as the strip moves along the conveyor rollers. It has beenfound that with the shield so positioned, it is very effective formaintaining the heat in the strip thus preventing the temperature of anyportion of the strip from falling below 1550 F.

In another aspect of the invention, differential cooling of the bar issubstantially reduced by arranging either the reflector or the heatshield in sections along the length of the strip with adjacent sectionsof the reflector or shield covering adjacent areas of the strip, and byproviding means for independently moving the sections into or out of thevicinity of the strip thereby independently controlling the cooling ofthe various areas of the strip to reduce differential cooling amongthese areas. The sections are generally moved into the vicinity of thestrip in sequence in the same direction as the strip is advancing sothat areas towards the tail of the strip are covered for a longer timethan areas towards the head. This counteracts the tendency of the tailend to cool more than the head end due to the longer exposure of thetail before entering the finishing train.

In yet another aspect of the invention, skid marks formed in the stripare substantially eliminated. This is accomplished by moving thesections of the reflector or heat shield into the vicinity of the stripover the skid marks as the marks advance along with the strip. Thus theskid marks, which are areas of the strip which are cooler than adjacentareas on either side of the skid marks, cool less than their adjacentareas, thus resulting in substantial equalization of the temperature inthe skid mark areas and the adjacent areas so that the skid marksdisappear or are at least less pronounced.

This sequence of moving the sections into (and out of) the vicinity ofthe strip so that the skid marks, but not their adjacent areas, arecovered can be superimposed on the general sequence of moving thesections into the vicinity of the strip in the direction of advance ofthe strip to differentially shield the tail end more than the head end.

To control the movement of the sections into and out of the vicinity ofthe strip, heat sensing devices can be arranged along the conveyor inthe conventional manner to sense the temperature of the strip as itadvances past the heat sensing device. Each sensing device controlsassociated circuitry which is activated according to whether thecorresponding area of the strip is above or below the correcttemperature at the heat sensing device. If the strip is above thecorrect temperature at a particular device, the appropriate section willbe moved away to allow that area of the strip to cool at a greater rate.If the strip is below the correct temperature at the heat sensingdevice, the appropriate section will be moved in to cover that area andreduce the rate of cooling.

The means to move the sections of the reflector or heat shield into orout of the vicinity of the strip can conveniently be an air cylinder andpiston. The free end of the piston can be attached to the section, andthe section hinged along one edge. As the piston moves in or out, thesection pivots away from or towards the strip.

In drawings which illustrate embodiments of the invention,

FIGURE 1 is a plan view of a semi-continuous hot strip mill employing aheat shield according to the invention,

FIGURE 2 is a side elevation of part of a hot strip mill employing adifferent embodiment of a heat shield of the invention,

FIGURE 3 is a perspective of one heat shield section of the type shownin FIGURE 1, and

FIGURE 4 is a perspective view of one section of a reflector accordingto the invention.

Referring particularly to FIGURE 1, a 5 ton slab of steel (not shown) 10feet long by 37 inches wide by 6 inches thick, is heated in a furnace 10to a temperature of about 2200 F. It is supported for part of its travelthrough the furnace on Water cooled skids. The hot slab then slides downthe runners or skids 11 from the furnace 10 onto a conveyor 12 havingrollers 13. The slab is moved along the conveyor 12 to a roughing mill14 through which the slab makes 3 to 5 or more passes back and forth tobe rolled into a strip 16 (FIGURE 2) approximately of an inch thick andfeet long. On each pass forward through the roughing mill 14, the slabis blasted with a jet of water from a de-scaler 17 to remove thecontinually forming scale from the slab. The jet is directed against theforward direction of the slab so that most of the scale flies offbackwards although a certain amount of the scale does fly forward. Thestrip 16 advances from the roughing mill 14 along a delay table 18 onconveyor rollers 19. The strip passes through a crop shear 22 located atthe end of the delay table. After passing through the crop shear 22, thestrip passes through a scale breaker 23 where scale which formed on thestrip while it was on the delay table is removed, and then the stripenters a finishing train 24. The finishing train 24 consists of sixstands of rollers 26 (only the first two of which are shown) where thestrip is rolled down to its final gauge, which may be, for example of aninch in thickness. The final gauge strip may be 800 to 1800 feet long(or longer in some mills), and is coiled up in coilers 27 into largecoils or rolls ready for further use.

A heat shield 28 is formed of 10-foot sections 29 (best shown in FIGURE3) arranged end to end along the delay table 18, parallel to the delaytable and about 5% inches above it so that the shield 28 is about 4 /2inches from the upper surface of the hot strip 16 as it moves along thedelay table. Extending back from the crop shear 22 towards the roughingmill 14, the heat shield is in the form of individual sections 29 aboutfeet long and arranged end to end to cover the delay table 18 back adistance from the crop hear 22 equal to the length of the longest stripemerging from the roughing mill. The shield sections 29 moveindividually into and out of the vicinity of the strip when the hotstrip mill is in operation, as Will be described later with particularreference to FIGURE 3.

From the point on the delay table where the individually movablesections 29 endthat is, from the point a distance back from the cropshear equal to the length of the longest stripto a point partway back tothe roughing mill,.the hield is in the form of a continuous tunnel 31.The tunnel 31 does not extend all the way back to the roughing millsince some of the scale which is blasted off the slab by the de-scaler17 flies forward and would build up on top of the tunnel. The tunnel 31can be made up of individual sections 29, but during operation of themill these sections may not move. However, it is convenient for cleaningof the tunnel and maintenance of the rollers to make it in sectionswhich can be individually moved away from the delay table.

Referring particularly to FIGURE 3, each section 29 has a steel plate 32which may be stainless steel and corrugated to allow for expansion andcontraction, and a backing 33 of asbestos. The plate and backing arecantilevered by two cantilever members 34 secured at one end to theplate 32 by fastening members 36. The other ends of the cantilevermembers 34 are secured to the upper ends of two outer uprights 37 spacedapart by two cross members 38. The lower ends of the two outer uprights37 are pivoted on hinge pins 39 journaled in supports 41 secured to abase plate 42. Two inner uprights 43 are fastened to the cross members38. An air cylinder 44 having a piston 46 is pivotally mounted on asupport 47 by pin 48. One end of a connecting rod 49 is fastened to thepiston 46, and the other end of the rod 49 is pivotally connected by pin51 to the two inner uprights 43. Two stops 52 extend downwardly from theplate 32 and rest on abutments 53 when the shield section 29 is in itsdown position, holding the plate 32 substantially horizontal. When thepiston 46 is drawn into the cylinder 44, the shield section 29 tilts upabout the hinge pins 39. The section is mounted with its tilt axis (i.e.the axis of the hinge pins 39) parallel to the length of the delay table18, and so that when the section is in its down position, the plate 32is positioned about 4 /2 inches above the hot steel strip as the stripmoves along the delay table 18.

In the embodiment shown in FIGURE 2, the shield sections 29 are of aslightly different construction which will not be described here indetail, it being obvious to one skilled in the art what sort ofconstruction would be suitable. The essential feature of this embodimentis that the sections tilt about an axis transverse to the length of thedelay table 18.

In FIGURE 4 a reflector 60 is shown which is an alternate apparatus tothe heat shield described in connection with FIGURES 1 to 3. Thereflector 60 is positioned over the delay table 18 so that the strip(not shown) passes under the reflector when advancing along the rollers19 of the delay table towards the finishing train. The reflector 60 isconcave towards the delay table 18 so that heat radiated from the stripas it passes under the reflector will be effectively focussed backtowards the strip. The reflector can advantageously be made of aluminum,in which case care must be taken to ensure that the reflector is placedfar enough from the strip so as not to melt. About 40 inches has beenfound satisfactory.

As was the case with the heat shield, the reflector can be made in theform of a continuous tunnel and also in the form of sections,conveniently about 10 feet in length. The sections can be moved into andout of the vicinity of the strip in the same way as was described inconnection with the heat shields of FIGURES 1 to 3.

I claim:

1. In a hot strip mill, wherein a strip is transported along a delaytable from a roughing mill to a finishing mill, the method of reducingdifferential cooling effects on the strip while it is on the delay tablewhich comprises the step of passing the strip below a series of heatshields movably disposed above said delay table and adjusting the heightof each of said heat shields above the strip in accordance with thetemperature of the strip beneath each said shield.

2. In a hot strip mill; a roughing mill, a finishing mill, and a delaytable extending theerbetween, said delay table having first and secondparts, a heat reflector disposed above the first part of the delaytable; a plurality of heat shield units movably disposed above thesecond part of the delay table; and means for selectively bringing eachof said units into proximity with the surface of said second part.

3. In a hot strip mill, apparatus "for equalizing the temperaturedistribution along a hot strip having relatively hot and relativelycooler regions, comprising means for selectively reducing heat loss dueto radiation from the relatively cooler regions of said strip; saidmeans including a heat shield divided into a plurality of sectionsdisposed along the length of the strip; and actuating means forindependently moving each of said sections into or out of proximity withsaid strip.

4. Apparatus as claimed in claim 3, further comprising means forcontrolling said actuating means in response to the temperature ofvarious areas of the strip.

5. Apparatus as claimed in claim 3, in which the heat shield comprises asteel plate having heat insulating material on a surface thereof remotefrom said strip.

6. Apparatus as claimed in claim 3 in which said actuating meanscomprises an air cylinder and piston arrangement for each of saidsections.

7. In a hot strip mill wherein a strip is transported along a delaytable from a roughing mill to a finishing mill, apparatus for reducingdifferential cooling effects on said strip while on said delay table,said apparatus comprising a series of heat shields movably disposedabove said delay table; a plurality of heat reflectors and means forselectively bringing each of said reflectors into proximity with saidstrip so as to reduce heat loss by radiation from'the region of thestrip below said reflector.

References Qited by the Examiner UNITED STATES PATENTS 1,347,917 7/ 1920Sheperdson 72-202 1,676,176 7/1928 Biggert 72202 1,946,971 2/ 1934Harter 2633 1,959,095 5/1934 Etherington 72202 2,564,708 8/1951 Mochel26350 2,728,387 12/1955 Smith 263-50 CHARLES W. LANHAM, PrimaryExaminer. H. D. HOINKES, Assistant Examiner.

1. IN A HOT STRIP MILL, WHEREIN A STRIP IS TRANSPORTED ALONG A DELAYTABLE FROM A ROUGHING MILL TO A FINISHING MILL, THE METHOD OF REDUCINGDIFFERENTIAL COOLING EFFECTS ON THE STRIP WHILE IT IS ON THE DELAY TABLEWHICH COMPRISES THE STEP OF PASSING THE STRIP BELOW A SERIES OF HEATSHIELDS MOVABLY DISPOSED ABOVE SAID DELAY TABLE AND ADJUSTING THE HEIGHTOF EACH OF SAID HEAT SHIELDS ABOVE THE STRIP IN ACCORDANCE WITH THETEMPERATURE OF THE STRIP BENEATH EACH SAID SHIELD.